Tonight – September 1, 2016 – Neptune comes closest to Earth for the year. It reaches opposition – when it is most opposite the sun from Earth for this year – on September 2. By closest, we don’t mean close. Neptune lodges in the outskirts of our solar system and at opposition lies 29 times farther away from Earth than Earth lies from our sun.
Neptune is said to be at opposition – opposite the sun in Earth’s sky – whenever our planet Earth in its orbit passes between the sun and Neptune. That’s what’s happening over the next couple of days.
Because we’re more or less between it and the sun around now, Neptune is rising in the east around the time of sunset, climbing highest up for the night around midnight and setting in the west around sunrise.
As viewed from Earth now, this world is in front of the constellation Aquarius the Water Carrier.
Opposition is a special event. When any planet outside of Earth’s orbit is at or near opposition, Earth comes closest to that planet for the year, and that planet, in turn, shines most brightly in our sky. Even at opposition, however, Neptune, the eighth planet outward from the sun, is not all that close and it’s not all that bright.
In fact, Neptune is the only major solar system planet that’s absolutely not visible to the unaided eye. This world is about five times fainter than the dimmest star that you can see on an inky black night. You’ll need binoculars (at least) and a detailed sky chart to see Neptune in front of the constellation Aquarius.
Even at that, it’ll only look like a faint star. Many sky watchers will find the faint star Lambda Aquarii with the unaided eye and then star-hop to Neptune.
Neptune, the fourth-largest planet, is just a touch smaller than Uranus, the third-largest. You’d have to line up four Earths side by side to equal the diameter of either planet.
Okay, so it’s unlikely you’ll see Neptune unless you have optical aid and a detailed star chart. But there are four other planets in the evening sky now. First, look for the Saturn and Mars, as shown on the chart below.
It’s pretty easy to spot them anytime during the evening hours, that is, from sunset until a few hours later, as seen from around the globe.
Now here are two more planets, which are a little tougher to see because they’re near the sunset. You have to look outside shortly after the sun goes down. On the other hand, Venus and Jupiter are the two brightest planets. Plus the moon is moving past them, beginning on September 2.
Bottom line: On September 1, 2016, the Earth is about to swing in between the sun and Neptune. On this day, Neptune comes closest to Earth and shines at its brightest for the year. Yet … you’ll still need good binoculars on a tripod or a telescope to spot Neptune.